This year in the U.S. close to 800,000 people will suffer a stroke, making it the third leading cause of death. A particularly devastating form of this illness, subarachnoid hemorrhage (SAH), represents 25% of life years lost among all forms of stroke due to the relatively young age of its victims. A SAH occurs when large vessels in the brain rupture and bleed into the fluid filled space surrounding the brain. Patients recovering from SAH often develop delayed neurological deficit due to ischemia, killing a significant number, and leaving survivors with permanent disability. The pathogenesis of this injury is not well understood, and even though several promising mechanisms of delayed neurological deficit have been studied in great detail, treatments developed from this work have had limited success in clinical trials. A recently documented phenomenon that correlates with delayed neurological deficit is delayed microvascular spasm (dMVS). Until now, the technology available to document dMVS has only been able to do so at very low resolution. Using a novel in vivo optical imaging modality called optical microangiography (OMAG), we have been able to directly visualize microvessels within the mouse cortex and definitively identify dMVS after SAH. We propose to use neurobehavioral tests, histology, and biochemistry in addition to OMAG to explore a potential therapeutic target to prevent dMVS. Flow in brain microvessels is controlled by cytochrome P450 metabolites of arachidonic acid called epoxyeicosatrienoic acids (EETs). EETs are inactivated by the enzyme soluble epoxide hydrolase (sEH). Our preliminary data show that sEH knockout mice are protected from dMVS after SAH. We will test the hypothesis that SAH induces upregulation of sEH in cerebrovascular endothelium, leading to a decrease in bioavailable EETs and the development of dMVS and neurological deficit. In Specific Aim 1, we will test whether genetic deletion and pharmacological inhibition of sEH activity prevent dMVS and neurological deficit in mice after SAH. We will further characterize dMVS and neurological deficit after SAH using OMAG, neurobehavioral examination and histology. Finally, we will use transgenic mice with endothelial overexpression of sEH and the EETs biosynthetic enzyme P450 2J2, to confirm the role of endothelial sEH in modulating dMVS after SAH. In Specific Aim 2, we will measure changes in sEH expression and EETs within brain microvessels after SAH by Western blotting, real-time quantitative PCR, and mass spectrometry (LC-MS/MS). Additionally, we will test whether the upregulation of sEH after SAH is dependent on angiotensin signaling, which is known to be increased following SAH. This research aims to identify a novel therapeutic target for prevention of dMVS and delayed neurological deficit caused by SAH.